THE  PROPERTIES  OF  MANGANESE 
STEELS  CONTAINING  ONE  TO  FIVE 
PER  CENT  MANGANESE 


HARRY  THEODORE  WARREN 


THESIS 


FOK  THE 


I ) E G R K E O F B A G HELO  R O F SCI  E NGE 

IN 

CHEMISTRY 


COLLEGE  OF  LIBERAL  ARTS  AND  SCIENCES 


UNIVERSITY  OF  ILLINOIS 


1 922 


) 322 
YY2  5 


UNIVERSITY  OF  ILLINOIS 

May  31  j iq2 

THIS  IS  TO  CERTIFY  THAT  THE  THESIS  PREPARED  UNDER  MY  SUPERVISION  BY 

Harry  Theodore  Warren 

ENTITLED.  Tiie_. Exopua  r_t  i Slq  _ _Qf— Mangiar-e  aa  - -Sta.e  la  _ -Cent  ai  nlng.  _ One.  _ to — 
Five  Per  Cent  Manganese" 

IS  APPROVED  BY  ME  AS  FULFILLING  THIS  PART  OF  THE  REQUIREMENTS  FOR  THE 


DEGREE  OF Ba ch  el o r _ of _ _S c_i_e n_c  e _ in _ _C h em  i_s  try. 


500286 


Digitized  by  the  Internet  Archive 
in  2015 


https://archive.org/details/propertiesofmangOOwarr 


HISTORICAL 


The  first  work  on  manganese  steels  was  apparently  con- 
fined to  those  steels  containing  less  than  two  and  one-half 
per  cent  manganese.  This  was  carried  on  in  France  by  the  Terre 
Noire  Company,  which  also  made  high-grade  ferromanganese  for 
the  manufacture  of  these  steels.  Sir  Robert  Hadfield  first 
produced  what  is  now  known  as  manganese  steel.  In  1838  he  pub- 
lished a paper  on  the  subject,  summarizing  his  previous  papers. 
He  made  steels  containing  from  nine  to  fifteen  per  cent  manga- 
nese, and  devised  a satisfactory  heat  treatment  for  them.  He 
found  that  quenching  in  cold  water  from  900-1100°  C. (lemon- 
yellow)  produced  steels  of  great  toughness.  One  steel,  contain- 
ing 13*75  per  cent  manganese  and  0.85  per  cent  carbon,  when 
quenched  thus,  had  a tensile  strength  of  145,000  pounds  per 
square  inch,  and  an  elongation  of  50.7  per  cent  in  eight  inches. 
Though  too  hard  for  impression  by  a drill  press,  the  steel 
could  be  dented  with  a hammer.  Comparatively  small  deformations 
produced  a permanent  set.  It  is  noteworthy  that  Hadfield* s 
steels  were  all  rather  high  in  carbon,  some  containing  as  much 
as  1.5  per  cent.  He  could  not  make  low-carbon  steels  with  the 
ferromanganese  available. 

The  manganese  3teels  containing  less  than  three  per  cent, 
are,  according  to  Sauveur,  pearlitic,  while  further  additions 
of  manganese  and  carbon  produce  martensite,  and  finally  austen- 
ite. Hoyt  states  that  the  structure  of  the  intermediate  steels 


-2- 


is  troostite,  instead  of  martensite,  when  the  carbon  content 
is  greater  than  0.30  per  cent.  The  oearlitic  manganese  steels 
( 1 to  3 per  cent  manganese)  are  generally  ignored  by  manufac- 
turers, for  two  reasons:  first,  the  belief  that  these  steels 
are  too  brittle,  and  second,  the  difficulty  of  manufacture  of 
low-carbon  manganese  steels.  The  belief  that  these  steels  are 
brittle  is  the  result  of  Hadfield’s  statement  that  they  are 
hopelessly  so.  Since  this  time,  however,  some  metallurgists, 
notably  G-uillet,  have  found  that  this  applies  only  to  manganese 
steels  of  high  carbon  content  cooled  rapidly.  Low-carbon  pearl- 
itic  manganese  steels,  slowly  cooled,  are  not  at  all  brittle. 
Until  quite  recently,  it  has  been  difficult  to  manufacture 
such  steels,  because  the  only  f erromanganese  available  was  that 
made  in  the  blast  furnace.  This  f erromanganese  has  a high  car- 
bon content.  Nov;,  however,  f erromanganese,  low  in  carbon,  is 
successfully  manufactured  in  the  electric  furnace  or  by  the 
thermit  process.  The  manufacture  of  these  steels  should  there- 
fore be  given  more  attention.  Another  possibility  worthy  of 
investigation  is  that  of  casehardening  articles  made  of  these 
steels,  to  produce  austenitic  cases  while  the  cores  remain 
pearlitic. 

Manganese  forms  a continuous  series  of  solid  solutions 
with  iron,  and  this  tendency  is  not  lost  even  in  the  presence 
of  carbon,  though  manganese  does  form  a carbide,  Mn^C.  At  high 
temperatures,  this  carbide  forms  a continuous  series  of  solid 
solutions  with  manganese.  The  exact  chemical  composition  of 
the  carbide  present  in  manganese  steels  is  undetermined.  Arnold 


-3- 

and  Read  made  an  exhaustive  study  of  the  chemical  relations 
between  iron,  manganese , and  carbon,  using  a series  of  steels 
of  the  same  carbon  content,  between  0.3  and  1.0  per  cent,  and 
manganese  contents  varying  from  0.41  per  cent  to  19.59  per  cent. 
They  were  unable  to  ascertain  definitely  whether  the  double 
carbides  we  re  really  compounds  or  merely  mixtures  of  Fe-jC  and 
Mn^C.  However,  they  reported  that  there  appeared  to  be  only  a 
mixture  in  steels  containing  4.98  per  cent,  or  less,  of  manga- 
nese, while  higher  percentages  of  manganese  seemed  to  produce 
the  double  carbides  ^Fe-^C. Mn^C  and  2Fe3C. Mn-^C.  The  carbide, 
possibly  one  of  these  compounds,  that  appears  in  high-carbon, 
high -manganese  steels,  as  cast,  is  apparently  a source  of  weak- 
ness. It  can  be  seen  as  membranes  around  the  polyhedral  crystals 
of  austenite,  and  also  segregated  in  chunks.  When  the  steel 
has  been  quenched  in  water  from  a high  temperature,  and  thus 
greatly  toughened,  the  structure  shows  no  carbide.  The  quench- 
ing prevents  the  carbide  from  separating  out,  and  leaves  it  in 
solution. 

Manganese  depresses  the  critical  temperatures  of  steel, 
so  that  the  forms  that  exist  only  at  high  temperatures  in  car- 
bon steels  are  present  as  normal  (though  not  necessarily  stable) 
constituents  of  manganese  steels  at  ordinary  temperatures. 
Manganese  steels  cool  more  rapidly  than  carbon  steels,  and 
have  a greater  contraction,  which  tends  to  cause  piping  and 
settling  in  the  molds.  The  specific  gravity  of  manganese  steels 
is  somewhat  greater  than  that  of  carbon  steels. 


-4- 


EXPERI MENTAL 

It  was  proposed  to  investigate  the  properties  of  manganese 
steels  with  manganese  content  of  one  to  five  per  cent,  with  a 
view  to  finding  whether  the  brittleness  could  be  eliminated  in 
these  steels.  If  these  steels  can  be  made  malleable,  they  will 
be  valuable  commercially. 

It  was  desired  to  study  first  the  carbide  of  manganese, 
and  the  absorbtion  of  carbon  by  manganese.  The  absorbtion  of 
carbon  by  manganese  was  to  be  brought  about  by  casehardening 
some  pure  manganese.  Attempts  were  made  to  determine  the  crit- 
ical point  for  manganese,  in  order  to  find  the  best  temperature 
for  the  casehardening.  Some  pure  manganese  was  ground  up  and 
put  into  a small  fireclay  crucible.  A hole  was  bored  in  the 
side  of  the  crucible  and  the  thermocouple,  enclosed  in  a silica 
tube,  was  put  into  the  metal  through  this  hole.  The  metal  was 
then  heated  in  an  electric  furnace.  Two  different  attempts  were 
made  in  this  way,  but  no  sign  of  a critical  point  appeared  on 
either  cooling  curve,  and  though  there  was  an  indication  of  a 
critical  point  at  1009°  G.  on  one  heating  curve,  no  such  indi- 
cation appeared  on  the  other  heating  curve.  In  the  two  attempts, 
the  highest  temperatures  reached  were  1025°  C.  and  1062°  C. , 
respectively.  In  each  case,  the  metal  was  badly  oxidized,  and 
fused  into  a solid  mass.  The  experiment  was  repeated,  and  this 
time  a steady  current  of  CO2  was  kept  flowing  through  the  fur- 
nace, to  prevent  oxidation  of  the  manganese.  The  metal  was 
again  found  to  be  completely  fused,  however,  and  no  indication 
of  a critical  point  was  observed.  The  highest  temperature  was 
1025°  C. 


-5- 


On  the  fourth  attempt,  nitrogen  was  substituted  for  C02,  and 
the  temperature  reached  was  1049°  G.  The  metal  was  slightly 
fused.  An  indication  of  a critical  point  at  1021°  G.  was  found 
on  heating,  but  no  point  was  obtained  oh  cooling.  For  the  last 
attempt,  the  manganese  was  used  in  a different  form.  Two  pieces 
of  manganese  (about  80  to  100  grams  each),  each  with  one  flat 
surface,  were  used.  A groove  was  ground  in  each  of  these  two 
surfaces  and  the  two  pieces  were  wired  together  in  such  a way 
that  the  grooves  were  opposite  each  other  and  formed  a hole 
into  which  the  thermocouple  could  be  inserted.  The  metal  was 
heated  to  1050°  C. , and  again  no  critical  point  was  found  on 
cooling,  though  on  heating,  a slight  temperature  pause  was  found 
at  1013°  G. 

No  more  attempts  were  made  to  determine  the  critical  point 
for  mangahese,  but  1050°  G.  was  selected  as  the  best  temperature 
for  casehardening.  The  first  casehardening  mixture  used  con- 
sisted of  two  parts,  hy  weight,  of  BaCO^,  two  parts  of  powdered 
charcoal,  and  three  parts  of  bone  meal.  This  was  not  quite 
satisfactory,  and  a mixture  of  sixty  per  cent  charcoal  and 
forty  per  cent  BaCO^  was  substituted,  with  good  results.  At 
first  the  casehardening  was  done  in  small  fireclay  crucibles, 
with  a cover  of  alundum  cement  paste.  The  alundum  cracked  a 
little  around  the  edges  and  when  the  crucibles  were  kept  at 
1050°  C.  for  more  than  two  or  three  hours,  a great  deal  of  the 
charcoal  burned  out.  An  iron  pipe,  eight  inches  long  and  two 
and  one-half  inches  in  diameter  inside,  was  then  substituted 
for  the  crucibles.  One  end  of  the  pipe  was  closed  with  an  iron 
top;  alundum  cement  was  used  to  stop  the  other.  Not  enough 


-6- 


carbon  burned  out  to  prevent  good  carbonization  when  the  pipe 
was  used.  Pieces  of  Armco  iron,  pure  manganese,  and  .20  carbon 
steel  were  put  into  the  casehardening  mixture  together.  The 
manganese  showed  the  best  carburization  of  the  three.  Casehard- 
ening was  tried  at  925°  C. , but  the  carburization  was  not  as 
complete  at  this  temperature  as  at  1050°  C.  The  duration  of  the 
process  varied  from  two  to  six  hours,  the  latter  time  giving 
the  best  results.  Figure  2 shows  a good  example  of  casehardened 
manganese. 

Some  manganese  carbide  was  next  made  by  melting  some  ground 
manganese  in  a fireclay  crucible  with  powdered  charcoal.  At 
a temperature  of  131C°  C. , the  molten  metal  ran  through  the 
crucible.  Enough  of  it  was  saved  to  make  a small  button.  This 
button  could  be  ground  on  an  emery  wheel  without  cracking,  but 
it  broke  into  bits  under  a,  pressure  of  500  kilograms  in  a Brin- 
ell  machine.  The  structure  of  the  material  was  more  fine-grain- 
ed than  that  of  the  case-hardened  manganese.  A second  quantity 
was  made  up,  and  had  similar  properties.  A large  gas  pot-furnace 
was  used  to  melt  the  metal.  Photographs  of  this  carbide  are 
shown  in  figures  3 and  4. 

Attempts  were  now  begun  to  make  manganese  steels  of  from 
3 to  4.5  per  cent  manganese.  The  first  sample  consisted  of 
112  grams  of  0.30  carbon  steel  and  5 grams  of  pure  manganese, 
which  would  give  a steel  with  0.29  per  cent  carbon  and  4.27 
per  cent  manganese.  A graphite  crucible  was  used,  in  a gas  pot- 
furnace.  The  crucible  and  the  sides  of  the  furnace  were  melted, 
but  the  steel  was  not.  A large  oil  spray  pot-furnace  was  then 
tried.  On  the  second  attempt,  a melt  was  obtained  and  the  steel 


-7- 

was  poured  into  a small  steel  mold.  On  cooling,  however,  the 
steel  stuck  to  the  mold,  and  could  not  even  he  hammered  out. 

The  mold  was  sawed  in  two,  so  as  to  get  a surface  of  the  man- 
ganese steel  to  polish  and  photograph.  No  tests  could  he  made 
on  this  steel. 

% 

Another  quantity  of  steel  and  manganese  was  melted  up,  in 
the  proportions  necessary  to  give  a steel  containing  3*3  per 
cent  manganese  and  0,28  per  cent  carhon.  This  steel  wa.s  poured 
into  a sand  mold.  The  sand  was  evidently  wet,  and  the  casting 
was  so  full  of  holes  as  to  he  quite  worthless.  The  steel  seem- 
ed to  he  extremely  hard  and  brittle. 

A quantity  of  .30  carhon  steel,  .10  carhon  steel  and  pure 
manganese  was  melted  to  obtain  a steel  of  3*4-9  per  cent  man- 
ganese and  0.236  per  cent  carhon.  A lined  graphite  crucible 
and  the  oil-spray  pot-furnace  were  used.  Two  small  sound  cast- 
ings were  obtained,-  one  in  a steel  mold,  and  one  in  sand.  A 
small  surface  was  polished  on  the  steel  mold  casting  and  the 
photograph  shown  in  figure  5 was  obtained.  The  steel  apparently 
contains  a great  deal  of  carbides.  This  bar  was  then  heated 
to  orange  heat  and  hammered.  It  was  fairly  malleable,  and  was 
hammered  to  about  half  Its  original  thickness.  After  cooling 
it  could  be  sawed  in  two  with  a hack  saw.  Figure  6 shows  a 
photograph  of  this  forged  steel,  with  the  flattened  grain  struc- 
ture resulting  from  forging.  One  piece  of  the  bar  was  then 
heated  to  1200°  G.  for  thirty  minutes  and  quenched  in  oil.  The 
structure  v/as  found  to  be  mostly  martensite,  with  some  austen- 
ite. (Figures  7 and  8) 

The  sand  casting  v/as  too  hard,  as  cast,  to  be  sawed  before 


-3- 


heat  treatment.  It  was  packed  in  calcium  oxide  in  the  pipe 
used  in  the  casehardening  work.  The  pipe  was  sealed  with  alun- 
dum  cement.  It  was  heated  in  a gas  muffle  furnace.  The  temper- 
ature was  raised  rather  slowly  to  7 00°  C. , and  then  rather  rap- 
idly to  1220°  C.  It  had  been  intended  to  go  only  aboutto  1150°  G. 
but  the  needle  of  the  pyrometer  stuck  at  1100°,  and  this  was 
discovered  and  remedied,  but  it  was  found  that  the  temperature 
was  1220°  G.  The  gas  was  shut  off,  and  the  furnace  opened  amid 
allowed  to  cool  to  6 50°  C.  The  furnace  was  then  closed  and  re- 
heated to  1050°  C.  The  temperature  was  kept  at  1025-1050°  for 
forty-five  minutes,  and  then  allowed  to  fall  very  slowly,  six 
hours  being  required  to  cool  to  450°  C.  The  casting  was  found 
to  have  been  softened  considerably  by  the  heat  treatment,  and 
could  be  sawed  in  two  fairly  easily.  The  microphotograph  (Fig. 9) 
shows  the  coarsely  crystalline  structure  of  normalized  low- 
carbon  steel,  with  needles  of  manganese  carbide. 

An  attempt  was  next  made  to  produce  a manganese  steel  con- 
taining 2,0  per  cent  manganese  and  .20  per  cent  carbon.  The 
oil-spray  pot-furnace  was  tried  first,  but  the  attempt  was  un- 
successful. A gas  furnace  was  then  used.  This  furnace  had  a 
half -inch  lining  of  alundum  cement  with  a three-inch  insulation 
of  Sil-O-Cel  between  the  lining  and  the  jacket  of  sheet  iron. 

The  flame  entered  a hole  just  above  the  bottom  of  the  furnace, 
from  a special  burner.  The  nozzle  of  this  burner  consisted  of 
a three-eighths  inch  iron  pipe,  inside  of  which  was  a smaller 
pipe.  Air  came  out  through  the  smaller  pipe,  and  gas  through 
the  larger  one.  The  pipe  which  conducted  the  air  to  the  nozzle 
was  heated  for  a distance  of  about  three  feet,  by  a battery  of 


-Q- 


gas  burners  (from  a dismantled  combustion  furnace).  The  first 
attempt  to  melt  the  steel  in  this  furnace  failed,  evidently 
because  the  gas  pressure  in  the  mains  was  too  weak.  The  second 
attempt  was  successful,  but  the  crucible  slipped  while  the  steel 
was  being  poured,  and  nearly  all  of  the  metal  splattered  out  of 
the  steel  mold. 

An  attempt  to  make  a steel  containing  4.18  per  cent  man- 
ganese and  .214  per  cent  carbon  failed  because  the  metal  was 
not  hot  enough  when  taken  out  of  the  furnace,  and  froze  while 
it  was  being  poured. 

At  this  time  another  furnace  was  built  of  firebrick.  The 
cavity  in  the  furnace  was  about  ten  inches  square,  and  the 
depth  was  nine  inches.  The  opening  was  made  almost  half-way  up 
from  the  bottom  of  the  furnace,  and  at  one  corner.  The  other 
three  corners  were  built  up  v/ith  alundum  cement,  to  give  a round- 
ed surface.  This  caused  the  flames  to  encircle  the  inside  of 
the  furnace  completely,  and  made  it  possible  for  the  crucible 
to  get  the  full  heat  without  being  placed  directly  in  the  full 
blast  of  the  flame.  This  arrangement  made  the  heating  of  the 
crucible  fairly  even  all  around,  and  reduced  the  tendency  of 
the  crucible  to  soften  and  give  way  on  one  side.  The  furnace 
was  unsuccessful  at  first,  however,  and  this  was  found  to  be 
due,  partly  at  least,  to  the  fact  that  the  opening  for  the 
flame  was  too  high  up.  The  flames  rose  after  entering  the  fur- 
nace, and  the  bottom  of a crucible  remained  comparatively  cool. 
(The  burner  was  the  same  as  used  for  the  circular  furnace  pre- 
viously described. ) Better  results  were  obtained  after  the 
furnace  was  rebuilt  with  the  opening  for  the  flames  close  to 


-10- 

the  bottom. 

A steel  containing  3*54  per  cent  manganese  and  .23  per  cent 
carbon  was  now  made  in  the  furnace  described.  It  was  cast  in  a 
sand  mold,  and  appeared  to  be  a sound  casting.  It  was  too  hard 
to  be  sawed  in  two  with  a power  hack  saw  before  heat  treatment. 

It  was  packed  in  CaO  in  the  pipe  used  for  previous  heat  treatments, 
and  heated  in  a gas  muffle  furnace  to  1080°  G.  It  had  been  intend- 
ed to  heat  the  steel  to  about  1200°  C.,  but  the  gas  pressure  was 
too  low  to  allow  a temperature  of  more  than  108C°  G.  to  be  reached. 
The  steel  was  then  cooled  to  750°  C. , and  reheated  to  1050°  C. 

It  was  kept  at  1050-1060°  G.  for  one  hour  and  five  minutes,  and 
then  cooled  very  slowly  in  the  furnace.  The  casting  could  then  be 
sawed  easily.  It  had  a Brinell  number  of  286.  When  it  was  sawed, 
it  was  found,  however,  that  it  was  too  full  of  holes  to  be  suit- 
able for  making  a test  bar. 

Three  giore  attempts  were  made  to  obtain  a casting  large 
enough  and  sound  enough  to  make  a test  bar,  but  all  of  them  fail- 
ed. It  was  found  that,  as  a rule,  the  gas  pressure  was  sufficient 
to  yield  the  necessary  heat  only  late  in  the  evening,  and  as  the 
furnace  was  being  used  by  another  man  also,  the  opportunities 
for  obtaining  a melt  were  few.  Also,  before  enough  experience 
was  gained  in  making  castings  to  give  satisfactory  techMque, 
the  school  year  was  ended. 

One  reason  why  the  castings  made  in  the  sand  molds  were 
unsound  was  that  no  "riser"  or  "feeder"  was  provided  for  in  the 
molds.  If  a bell-shaped  enlargement  is  made  at  the  top  of  the 
mold,  and  enough  metal  is  poured  in  to  fill  the  mold  to  the  top 
of  this  riser,  the  castings  are  far  more  likely  to  be  sound. 

The  holes  are  made  by  the  contraction  of  the  metal  on 


. 


- 

-11- 

freezing.  If  there  is  a supply  of  molten  metal  at  the  too  of 
the  casting,  it  will  he  drawn  down  by  the  shrinking  of  the  low- 
er part,  and  will  fill  up  the  holes  there. 

Owing  to  the  fact  that  no  casting  suitable  for  a test  bar 
was  made,  no  figures  showing  the  tensile  strength,  elastic  lim- 
it, elongation,  and  reduction  of  area  could  be  obtained.  The 
Brinell  hardness  was  not  taken  on  the  small  castings  made  early 
in  the  work,  as  they  were  really  only  preliminary  efforts,  and 
it  was  hoped  at  that  time  that  it  would  be  possible  to  make 
large  castings  from  which  all  necessary  significant  data  could 
be  obtained. 

CONCLUSIONS 

Should  this  investigation  be  continued,  the  work  already 
done  shows  that  two  essential  features  must  be  developed.  These 
are,  first,  crucibles  that  will  stand  melting  steel  and  slag 
up  to  about  1700°  C.,  and  second,  a furnace  of  capacity  to  melt 
steel  to  cast  an  ingot,  probably  three  inches  in  diameter,  and 
long  enough  to  permit  a sound  test  bar  to  be  cut  from  it. 

At  the  time  this  investigation  was  started,  work  was  begun 
on  an  electric  carbon  resistance  furnace  designed  to  give  a 
temperature  of  1800°  C.  Receipt  of  the  refractory  parts  of  this 
furnace  was  delayed  long  past  the  expected  date  of  delivery. 

The  furnace  was  assembled  and  the  test  runs  made  on  it  during 
the  last  few  days  of  the  semester.  No  time  was  available  for 
using  it  in  this  investigation,  as  was  the  intention  when  the 
work  was  started. 

Although  the  results  of  this  work  were  so  meagre,  they 


-12- 


give  noteworthy  indications  of  the  probability  that  steels 
containing  one  to  five  per  cent  of  manganese  can  be  freed  from 
brittleness  by  proper  heat  treatment.  This  result  was  apparently 
attained  in  this  investigation  by  heating  the  steel  to  a temper- 
ature above  1 100°  G. , cooling  to  about  700°  , reheating  to 
1025-1050°,  and  finally  cooling  very  slowly  in  the  furnace.  It 

has  at  least  been  shown  that  the  suoject  merits  further  and 
extensive  research. 

BIBLIOGRAPHY 

Journal  of  the  Iron  and  Steel  Institute,  1388,  No. 2, 

pp.  49, ff.  Manganese  Steels,  Sir  Robert  Hadfield. 

Sauveur-  The  Metallography  and  Heat  Treatment  of  Iron 

and  Steel. 

Hoyt-  Metallography. 

Von  Juptner-  Siderology. 

Hiorns-  Steel  and  Iron. 

Hoffman-  General  Metallurgy. 

Mars-  Die  Spezialstahle. 


x 100 

Fig. 3*  Manganese  carbide,  made 

by  melting  ground  manganese  with 

powdered  charcoal.  Etched  with 

HNO*. 

3 


x735 

Fig. 4.  Same  as  fig. 3.  Shows 
martensitic  structure  more 
plainly. 


x 1 00 


Fig. 5.  Steel  as  cast.  Manganese.  3*5  per  centJ  carbon, ,24  per 
cent.  Full  of  carbides.  Etched  with  KNO3. 


x 100 

Fig.  1.  Manganese,  etched  with 
HNO-z . Shows  usual  polyhedral 
structure  of  pure  metals.  Black 
spots  are  inclusions  of  impurities. 


xlQO 

Fig. 2.  Manganese,  casehardened 
for  six  hours  at  1050  G.  Etched 
with  HNOj. 


